Herein are described printing machines, and more specifically phase change ink apparatuses, with particular attention to pressure and transfix rollers for use in direct and offset printing or ink jet printing apparatuses. In certain embodiments, a single layer transfix pressure member can be used in high speed printing machines and can be used in combination with phase change inks such as solid inks.
Ink jet type phase change printing systems using intermediate transfer, transfix or transfuse members are well known, such as that described in U.S. Pat. No. 4,538,156. Generally, a printing or imaging member is employed in combination with a printhead. A final receiving surface or print medium is brought into contact with the imaging surface after the image has been placed thereon by the nozzles of the printhead. The image is then transferred and fixed to a final receiving surface by the imaging member in combination with a transfix pressure member, or in other embodiments, by a separate fuser and pressure member.
More specifically, one type of phase-change ink imaging process begins by first applying a thin liquid, such as, for example, silicone oil, to an imaging member surface. The solid or hot melt ink is placed into a heated reservoir where it is maintained in a liquid state. Once within the printhead, the liquid ink is ejected, typically through use of a piezoelectric transducer. Several rows of jets, for example four rows, can be used, each one with a different color. The individual droplets of ink are jetted onto the liquid layer on the imaging member. The imaging member and liquid layer are held at a specified temperature such that the ink hardens to a ductile visco-elastic state.
After depositing the image, a print medium is heated by feeding it through a preheater and into a nip formed between the imaging member and a pressure member, either or both of which may be heated. In certain apparatuses, a high durometer synthetic transfix pressure member is placed against the imaging member in order to develop a high-pressure nip. As the imaging member rotates, the heated print medium is pulled through the nip and is pressed against the deposited ink image with the help of a transfix pressure member, thereby transferring the ink to the print medium. The transfix pressure member compresses the print medium and ink together, spreads the ink droplets, and fuses the ink droplets to the print medium. Heat from the preheated print medium heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers or other like members, peel it from the printer member and direct it into a media exit path. On the other hand, in a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving web or substrate such as paper or cut paper.
To optimize image resolution, the transferred ink drops should spread out to cover a predetermined area, but not so much that image resolution is compromised or lost. The ink drops should not melt during the transfer process. To optimize printed image durability, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion. Finally, image transfer conditions should be such that nearly all the ink drops are transferred from the imaging member to the print medium.
The imaging member is multi-functional. First, the ink jet printhead prints images on the imaging member, and thus, acts as an imaging member. Second, after the images are printed on the imaging member, they can then be transfixed or transfused to a final print medium. Therefore, certain imaging members can provide a transfix function in addition to an imaging function. More specifically, a single drum surface transfers the image, spreads the ink droplets, penetrates the ink into the media, and controls the topography of the ink to increase paper gloss and transparency haze.
The process requires a delicate balance of drum temperature, paper temperature, transfix load, and drum and transfix roller materials and properties in order to achieve acceptable image quality. These combined requirements reduce the drum material possibilities mainly due to wear of weaker materials, which result in gloss and haze degradation. For most applications, a certain amount of gloss on a print is desired, but for some applications it is desirable to obtain either a very fine matte finish or a gloss finish.
In order to ensure proper transfer and fusing of the ink off the imaging member to the print medium, certain nip temperature, pressure and compliance are required. Unlike laser printer imaging technology in which solid fills are produced by sheets of toner, the solid ink is placed on the imaging member one pixel at a time and the individual pixels must be spread out during the transfix process to achieve a uniform solid fill. Also, in color printing machines the secondary color pixels on the imaging member are physically taller than the primary color pixels because the secondary pixels are produced from two primary pixels. Therefore, compliance in the nip is required to conform around the secondary pixels and to allow the primary pixel neighbors to touch the media with enough pressure to spread and transfer. A correct amount of temperature, pressure and compliance is required to produce acceptable image quality.
Currently, a typical transfix pressure roller for certain commercial products which produce up to 24 images per minute, comprises a substrate, a polyether-based polyurethane or a nitrile-butadiene rubber (NBR) intermediate layer having a modulus from about 40 to 120 MPa, and having a thickness of from about 1.0 to about 10.0 mm, and an outer layer comprising a polyester-based polyurethane or a nitrile butadiene rubber (NBR), having a modulus from 5 to 40 MPa, and a thickness of from about 0.1 to about 3.0 mm. Certain single layer transfix pressure rollers that produce up to 6 prints per minute comprise a substrate of a millable gum polyether-based polyurethane material having a modulus of about 70 MPa and a thickness of 2.6 mm.
The transfix pressure exerted at the nip in many prior machines is from about 500 to about 700 psi. However, more recent transfix pressure members must allow for exertion at the nip of from about 750 to about 4,000 psi for use in high-pressure, high-speed machines. Therefore, as the process speed goes up for high-speed machines, the size of the roller and the required pressure increases to enable high speed printing with desired image quality to achieve the same image quality. As the pressure requirement is increased, the design of the transfix pressure member requires that the layers on the member become thinner and harder for a given applied load on the member. As the layers become thinner and harder, the ability to keep uniform pressure across the nip, while maintaining the necessary nip profile for paper handling, becomes more and more difficult. In addition, the member sees reasonably high temperature variations, print liquids, and ink components, which could adversely affect its function and print quality.
In many solid ink jet and direct-to-paper applications, over and above the complex issues just described, duplex printing quality has been challenging. One problem is known as “ghosting” in which gloss patterns are created when the first printed side of the substrate contacts the pressure roller during duplex operation. When the previously applied ink comes into contact with the pressure roll, some of the oil that is in or on the ink from the initial spreading step transfers onto the pressure roller in the pattern of the first image. When this oil pattern on the pressure roller then comes in contact with the ink on the subsequent transfix step gloss patterns can be created.
One solution to this problem is related to the oil levels on the transfix and pressure rollers. In some solid ink jet processes, the transfix roller is oiled via contact with the imaging drum. In direct to paper systems there is no contact with the drum, so in some printing machines an oil maintenance unit is provided that applies oil directly to the pressure roll. This latter solution to direct printing systems adds cost and complexity.
What is needed is a pressure and transfix roller design that solves the problem of ghosting during duplex printing, particularly for solid ink or phase change printing machines, and for direct-to-paper printing machines. The roller design should also address oil level issues in both offset and direct printing applications.